This unit has been in for 3-4 months and worked fine. Took truck to Erie, PA and worked good going down there and all around. But Sunday afternoon it stopped tracking. It was in a while different area.

Tried going through all settings, adjusted location, reset it and everything else but it still isn't tracking right. It goes from 5 miles away to 20 miles away.When I come to a stop it seems to keep going for several seconds after.

This unit is not modded with exception to the bypass for the nav to work while moving.

I even got a extension cable to connect to the trucks factory xm antenna (yes it does the same as having the pioneer oem antenna) shows good connection.

I am having the same issues with a SUV we bought that has a 850 in it. Did you get a fix for this?

Anyone?

Anyone?

There's either something wrong with your installation or you have a hardware problem. In the Troubleshooting section of the manual, there are 10 possible Cause/Actions for your symptom. 

Is the VSS sensor connected and calibrated for your vehicle? This will be very helpful in dead reckoning when there are weak GPS signals. It also requires accurate inertial tracking. This can be interfered with by the angle of the headunit and possibly other nearby electronics.  Read the Positioning Technology section in the appendix of your manual. It might give you some other ideas on what is interfering with the GPS.

Also, are you using the vehicle's XM antenna?  While electrically, it may appear to work, there is a big difference in the technology. GPS antenna might have greater sensitivity required to receive as many as 12 discreet satellite signals. Try connecting the correct antenna and temporarily placing it in a location such as the roof. 

Sorry, there's no magic bullet. It's impossible for someone to diagnose this over the internet. Its possible you have a hardware problem with your unit. This is not a normal problem.